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Every now and then, I start my vehicle and I start driving it but the speedo doesn't move, the gear 'D' indicator on the dashboard display flashes and the car won't go more than about 20kph, it's still running smooth, sounds normal. Nothing I do ( extra revs ) helps and I just hope it gets me to my destination, which it always does. I turn the vehicle off and leave it for an hour then start it up and if I'm lucky, it resumes normally. If I'm not lucky it continues, it still does it when I restart it. It seems to happen after filling it up with petrol but not all the time. Mechanics are perplexed and I am worried about it. Any ideas?